1. Field of the Invention
This invention relates generally to covers for internal combustion engines, and more particularly to valve covers attached to a cylinder head.
2. Related Art
Valve covers for automotive and other internal combustion engine applications typically are made of die cast metal, metal stampings or molded thick-walled thermoset or thermoplastics materials. The covers typically have a perimeter flange formed with a series of spaced bolt holes that enable the cover to be bolted onto an associated cylinder head of the engine. Relatively tall gaskets are typically disposed between the flange of the cover and the cylinder head, and upon fastening the cover to the head, the gasket is compressed to form a seal. As such, the valve cover essentially floats on the head with the gasket being sandwiched therebetween. In this type of sandwiched construction, after extended use, the bolts can become relatively loose, for example, due to temperature changes, vibration, and loading, and thus, leaks can result.
The all-metal valve covers are common, but are costly and heavy. The thick-walled all-plastic valve covers can be less costly and can contribute to a reduction in weight, but often the structural and dimensional stability requirements call for such large wall thicknesses that the benefits offered by the plastics material are offset by the bulkiness of the product and space requirements taken up by the added wall thickness. The all-plastic valve covers also require added fasteners as compared to the all-metal valve covers in order to adequately clamp the seal or gasket to prevent leakage. As such, the span between adjacent bolt holes decreases and the number of fasteners increases as compared to metal valve covers, adding to the cost and weight of the valve cover assembly. Further adding to the cost of all-plastic valve covers is the incorporation of bolt isolators to avoid over tightening of the bolts.
Another known problem commonly encountered with valve covers is the generation of noise, vibration and harshness (NVH). As such, there have been continual efforts made in attempts to reduce NVH of these covers. Some known practices include forming an elastomeric bridge between a cover portion and a flange portion of the cover, however, these efforts have resulted in relatively high cost products, both in manufacture and in service. As such, in addition to solving the NVH problems, there are also continual efforts being made to reduce costs associated with the manufacture of covers and in servicing covers upon use. Often, in service, the entire cover needs to be replaced, regardless of the problem, thereby passing on relatively high costs to the end user and possibly resulting in waste. For example, in one known construction, the elastomeric bridge is constructed as one piece of material with the seal, and thus, if the seal becomes defective, it becomes necessary to replace the entire cover.